The present invention concerns a method for manufacturing an assembly comprising a hard plastic watch case and at least one push button having a stem and a flexible head projecting outside the case, the stem extending from the head through a lateral opening of the case, the flexible head being attached to the stem and the case and closing said opening in a sealed manner.
Push buttons of this type generally have a small travel, which destines them above all for controlling an electric contact in an electronic watch, for example a chronograph or a multi-function watch, but application thereof to a watch having a mechanical movement may also be envisaged.
Patent document GB-A-2 077 506 discloses a watch case having push buttons of this type, except that the stem does not completely pass or does not pass at all through the opening of the case. The stem and the external head of the push button are made of a single moulded hard plastic part. The head has a wall thin enough to be slightly flexible, to allow thus a small longitudinal travel of the stem and to act as return spring. The free end of the stem carries a conductive rubber plate which, when the push button is pressed, abuts two fixed contacts to connect them electrically. The periphery of the flexible head is in the shape of a cylindrical skirt which, in a first embodiment of the push button, is glued in a sealed manner against the wall of the opening of the case. In another embodiment, the moulded part is integral with the case, the skirt being directly connected to the case around the opening.
A push button head of this type must have a large enough diameter for the head to be flexible. This is possible in the case of the aforecited document because the push buttons are situated on an upper face of the case. Conversely, they could not be situated on the lateral wall of a wristwatch (the portion called the middle part), because the height of this wall is generally very limited. Moreover, repeated flexion of the hard plastic flexible head could eventually detach or split the skirt thereby affecting the sealing and operation of the push button.
The present invention aims to avoid the aforementioned drawbacks, as a result of a push button able to be made at low cost via moulding method and in such a way that the push button may be situated on the middle part of a wristwatch.
One aspect of the invention concerns an assembly of the type indicated hereinbefore, characterised in that the flexible head is made of an elastomeric material and is fixed in a sealed manner on an external surface of the case around the opening.
Due to the fact that it has very high capacity to be deformed elastically and is fixed on an external surface of the case and not in the opening, the elastomeric material of the flexible head allows a relatively narrow pusher to be made in the direction of the thickness of the watch, thus on the exterior of the middle part as in conventional chronograph watches with metal push buttons. Moreover, the elastomer head may advantageously be heat over moulded (also called duplicate moulding) onto the case, as will be described hereinafter, which assures solid and durable connection between the elastomeric material and the plastic material of the case. The elastomer head also has the advantage of absorbing accidental shocks on the push button, thereby also avoiding causing damage to the inside of the watch.
The stem may be made of a hard plastic material and in particular of the same plastic material as the case, which allows it to be moulded at the same time as the case. The stem may be made in a single piece with the case, by being connected thereto on one side by a flexible portion, or may be separate from the case being connected thereto solely by the elastomer head.
In another embodiment, the stem may be made of elastomeric material and be made in a single piece with the flexible head.
Another aspect of the invention concerns a method for manufacturing an assembly comprising a hard plastic watch case and at least one push button having a stem and a flexible head projecting outside the case, the stem extending from the head through a lateral opening of the case, the flexible head being attached to the stem and the case and closing in a sealed manner said opening, the method being characterised in that the flexible head is moulded by injecting an elastomeric material which connects itself in a sealed manner to an external surface of the case around the opening, and in that the method comprises the following steps:
(I) injecting the plastic material into a mould to form at least the case,
(II) modifying part of the mould in the region of the push button head, and
(III) injecting the elastomeric material in said part of the mould to form at least the flexible head so that said head is heat bonded to said external surface of the case.
An injection moulding method using two materials (also called two-shot moulding) is thus used, which is particularly efficient and economical because it only uses a single mould for making the watch case and its push button or buttons. Moreover, this method assures impeccable positioning of each push button stem with respect to the case and thus the movement.
In a particular way of implementing the method, the push button stem is made of the same plastic material as the case and is moulded during step (I).
In another way of implementing the method, the push button stem is made of the same elastomeric material as the flexible head and is moulded during step (III).